async def app(scope, receive, send):
    assert scope["type"] == "http"
    await send({
        "type": "http.response.start",
        "status": 200,
        "headers": [
            [b"content-type", b"text/plain"],
        ]
    })
    await send({
        "type": "http.response.body",
        "body": b"Hello, world!",
    })
